A Napier truck driver negotiating the steep Otira Viaduct for the first time probably experienced brake failure, sending him into the back of another truck, and then 30m over the side into the gorge and to his death, a coroner has ruled.

Piki Lewis Jones, 34, worked for St Giles Trading Company Ltd.

His wife told Coroner Christopher Devonport he was an experienced driver, although it was possibly his first trip from east to west.

At 6am on June 22, 2011, he was driving his truck and trailer, which was not fully loaded, down the viaduct when it broke through a barrier at Starvation Point, and plunged 30m on to the rocky riverbed below.

Mr Jones' wife said he had gained weight in recent years and had noticed his heart rate increasing and loss of breath when doing physical activity, such as getting in and out the truck.

His sister said that before his death he had joked about angina pain and "I am sure I am having mini heart attacks".

Police senior crash investigator Senior Constable Simon Burbery said at the time of the crash there was light fog and the road was wet. It was still dark.

Mr Jones pulled over at Deaths Corner at the top of the viaduct and let another trailer overtake him. That driver, Alexander Williams, noticed that Mr Jones was going faster as he descended, and wondered if he was trying to catch up with him.

"As the last of my trailer came around the corner I could see he had lost it and his truck came flying down and rammed the back of my trailer. Then he went over the barrier and down the bank," Mr Williams said.

"The impact to my trailer jolted me in the driver's seat of my truck. It was like a big bang."

The crash investigators found Mr Jones had crossed the centre line and hit the steel netting used to hold back rocks. The truck was travelling no less than 38-40km/h when it hit the barrier.

The trailer landed on the cab unit, which was crushed.

Truck driver Christopher Newton, who had been heading up the viaduct, said he heard the brakes on the other truck "working on a high rev range".

Autopsy revealed Mr Jones had significant heart disease. Blood tests showed he had smoked the equivalent of a single cannabis joint within three hours of death.

Constable Brett Currie found a plastic bag with cannabis at the accident scene.

The coroner said the truck was too damaged to determine what gear it had been in. If it had been in low third gear and the retarder on high, there would have been little or no need to apply the brakes.

The coroner heard expert evidence that the brakes on two of the axles were highly worn.

Mr Jones' log book showed false entries, which indicated he was not taking the required rest time; fatigue could not be ruled out as a factor.

The Coroner said what made Mr Jones speed up may never be known. However, experts agreed the brakes showed evidence of overheating, possibly during the crash trip.

He was "likely" not travelling in the right gear and had to apply his brakes in a way that let them to overheat and fail.

The coroner could not exclude fatigue or the effect of cannabis, or a medical event.

"I consider it most likely that lack of experience in driving down the Otira Gorge in the hours of darkness has resulted in driver error by failing to engage the correct gear before descent, and resulting in brake applications that has resulted in brake failure," coroner Devonport said.

An upgraded guardrail was installed in the Otira Gorge in August 2013.